1. Field of Invention
This invention relates generally to advanced orbit control and maintenance techniques for both individual satellites as well as for multiple satellites in a constellation, in which Modern Feedback Control is used for providing precise autonomous on-board navigation and control. The basic functions of this control system can place any satellite in any orbit position in a constellation, including the acquisition of the initial distribution for the constellation after satellite separation from launch vehicles, and can also maintain the constellation distribution, including station relocation and station keeping.
2. Description of Related Art
The orbital control of satellites, in both geostationary (GEO) and low-earth orbits (LEO), has primarily been ground-based. Orbit maintenance and station keeping have historically required involvement of Control Center personnel in all phases of operation. The computational burden for satellite control, including orbit analysis, maintenance and stationkeeping, has been on the ground computers. The ground computers provide both the off-line functions of orbit determination and maneuver planning as well as the on-line functions of commanding and telemetry processing.
Current geostationary satellite operations have evolved by taking advantage of the stationary nature of the satellite position relative to the ground stations. For example, the geostationary geometry provides a continuous window for ranging, tracking, and commanding, thereby minimizing the computational burden on the processors on-board the satellites. Low-earth orbit satellites have generally been equipped with more on-board processing capability than geostationary satellites to provide increased autonomy in navigation, command and control. This is because LEO satellites have intermittent ground station contacts of relatively short duration, resulting in limited ability to send commands to the satellites in real time.
The standard design for orbit control is based on the analysis of orbital mechanics, which provides the relationship among the orbital elements, orbit velocity changes and orbital behavior under the perturbing forces. Based on these relationships, orbit control is classified into individual control systems, for example, "orbit control", "orbit eccentricity control", "east-west position control", "drift or velocity drift control" and "orbit inclination control." These individual control laws reflect only partial relationships between individual orbit elements and control actions. However, while two-body orbit initial problem theory provides an analytical relationship between the six orbital elements, for example the semimajor axis (a), the eccentricity (e), the inclination (i), the right ascension of ascending node (.OMEGA.), the argument of perigee (.omega.), and the mean anomaly (M),and the initial state vector, this relationship is non-linearly coupled. When this coupling is neglected, the accuracy of any control system based on these individual models will be limited, and the efficiency of the control system will be low.
Various methods have been studied for control of satellite navigation.
U.S. Pat. No. 5,109,346 to Wertz discloses autonomous navigation control using Global Positioning Satellites (GPS) for orbit determination, and a method for providing orbital corrections. Because Wertz uses a non-feedback control system, this system is subject to unstructured uncertainty. Additionally, Wertz is limited to orbit and attitude determination. Furthermore, position finding using GPS is known, as described for example, in U.S. Pat. No. 4,667,203 to Counselman, III.
Historically, control systems were designed as proportional-integral-derivative (PID) compensators using a variety of frequency response techniques. However, the PID design requires trade-offs with conflicting design objectives such as the gain margin and closed-loop bandwidth until an acceptable controller is found. When the dynamics are complex and poorly modeled, or when the performance specifications are particularly stringent, more powerful control tools are necessary.
These more powerful design tools result in a higher level of satisfaction only if a solution exists to the problem being solved. Achieving both satisfactory performance limits and ascertaining the existence of a satisfactory controller involves using an optimization theory. Use of an optimization theory eliminates the need to search for solutions to problems for which there are no solutions. A further benefit of optimization is that it provides an absolute scale of merit against which any design can be measured. These more powerful design tools utilize modern advanced multivariable feedback control techniques.
It is an object of the present invention to utilize modern advanced multivariable feedback control techniques in the design of a navigation and control system.